1. Field of the Invention
The present invention relates to a boundary acoustic wave device preferably for use in a resonator, a band filter, etc., and to a method of manufacturing such a boundary acoustic wave device. More particularly, the present invention relates to a boundary acoustic wave device including a so-called three-medium structure in which first and second dielectric layers are stacked on a piezoelectric substrate, and to a manufacturing method thereof.
2. Description of the Related Art
Conventionally, in communication systems such as cellular phones, surface acoustic wave devices have been widely used as resonators and band filters. Also, attention has been focused on boundary acoustic wave devices instead of surface acoustic wave devices because the former does not require a package structure having a cavity. For that reason, boundary acoustic wave devices of various structures have been proposed.
WO2005/093949 discloses a boundary acoustic wave device 101 illustrated in a sectional view of FIG. 9. The boundary acoustic wave device 101 is a boundary acoustic wave device of a three-medium structure. In the boundary acoustic wave device 101, first and second dielectric layers 103 and 104 are stacked on a piezoelectric substrate 102. An electrode structure including IDT electrodes 105 is formed at an interface between the piezoelectric substrate 102 and the first dielectric layer 103.
When manufacturing the boundary acoustic wave device 101, firstly, the piezoelectric substrate 102 is prepared. The electrode structure including the IDT electrodes 105 is then formed on the piezoelectric substrate 102. The first dielectric layer 103 is then formed by magnetron sputtering. In this stage, the frequency, the acoustic velocity of a boundary acoustic wave, etc., are adjusted by adjusting a film thickness of the first dielectric layer 103. After the adjustment, the second dielectric layer 104 is formed.
The first dielectric layer 103 can also be formed by a method of bonding a dielectric wafer, which is prepared separately, instead of magnetron sputtering. However, it is difficult to uniformly bond the dielectric wafer. Further, there is a risk that the first dielectric layer 103 may be peeled off when a laminate body obtained as a mother substrate is cut. On the other hand, the first dielectric layer 103 can be easily and reliably formed by the manufacturing method described in WO2005/093949 because it utilizes the magnetron sputtering.
With the manufacturing method described in WO2005/093949, however, even when the frequency and the acoustic velocity of a boundary acoustic wave are adjusted before formation of the second dielectric layer 104, frequency characteristics, such as a resonant frequency and a center frequency, tend to vary in the manufactured boundary acoustic wave device 101 because the second dielectric layer 104 is formed after the adjustment.